Okay, let's just take a few hypothetical situations here and see what we can come up with. You specifically mentioned going from Mars to Jupiter, and want something realistic, but quick. Let's assume you have a 1g acceleration capability. How you do that is up to your game really. To make things a bit simpler, I'm going to have two modes of operation, which should bound pretty well how long it will take. I'm going to assume you want to stop there, and not just flyby. They are:
- Mars/Jupiter at closest approach. Will add in a bit of time to match the different relative velocities, and ignore inclination, etc.
- Mar/Jupiter at furthest apart. Will assume that one losses all Martian speed, heads straight to Jupiter, then gains all of the Jupiter speed.
These aren't quite accurate, but they are close enough. Okay, so here's a few key facts:
- Jupiter- Distance= 778,500,000 km, orbital velocity= 13.07 km/s
- Mars- Distance= 227,900,000 km, Orbital velocity= 24.125 km/s.
The time given a 0 starting speed, 0 ending speed, straight line, and half accel, half decel can be found by:
Okay, so, what do we have? Let's start with the maximum distance case. Distance is added, so 1,006,400,000 km. Plugging that in to the above formula, that gives us 453197 s, or about 125.9 hours. Adding in the relative speed difference will take another 3795 seconds, or a bit more than an hour. Thus, I'll say 127 hours should be sufficient.
How about the best case scenario? The distance is reduced to 550,600,000 km. Plugging it in to the same formula gives 335212 s, or 93 hours. The relative speed difference will be 1128 s, or only a fraction of an hour.
If you use a higher speed, the time will decrease by the same formula, just use a different value for a other than $9.8 m/s^2$. It seems likely that humans could accelerate at 3 g for some time, which would reduce the time by almost a factor of 2 to make this work. Beyond that, you need some sort of an inertial dampener.